The measurement accuracy of a force-measuring device, for example a balance based on electromagnetic force compensation or strain gauge technology (see [1], “Build your Quality on a Solid Foundation!”, company publication, Mettler-Toledo GmbH, January 2001, pages 14-15) is influenced by many factors which are described in [2], “Wägefibel” (Weighing Primer), Mettler-Toledo GmbH, April 2001.
Balances of all types are particularly susceptible to disturbances caused by mechanical influences such as vibrations or shocks, and they are therefore equipped with filters which serve to remove the disturbance-related signal portions. Methods in which signals produced by the measurement transducer of a force-measuring device are processed by means of digital filters are described for example in [3], US 2004/0088342 A1 and [4], U.S. Pat. No. 6,271,484 B1. Force-measuring devices with very high levels of measurement resolution wherein a measurement quantity is registered and converted into a digital signal by means of an analog/digital (A/D) converter further require a very precise reference, a reference voltage or a reference current. A variety of A/D converters, all of which are supplied with a reference voltage UREF, are presented in [5], U. Tietze, Ch. Schenk, “Halbleiterschaltungstechnik” (Semiconductor Circuit Design), 11th edition, 2d printing, Springer Verlag, Berlin 1999, pages 1043-1062. As can be easily understood for example from the so-called parallel method described in [5] on pages 1047-1048, the achievable resolution in the A/D conversion of an analog signal depends on the stability of the reference voltage UREF.
Measuring bridge circuits of the type used in balances based on strain gauge technology are described in [6], Albert O'Grady, Transducer/Sensor excitation and Measurement Techniques, Analog Dialogue 34-5 (2000). As shown in FIG. 8 on page 4 of reference [6], the signals that are present at the center taps of the measuring bridge can be converted into a digital signal by means of a sigma-delta A/D converter, wherein the voltages which are applied to the measuring bridge circuit are supplied to the sigma-delta converter as reference voltages (in regard to the working principle of measuring bridges see also [5], pages 1241 and 1243).
While conventionally generated reference voltages are normally adequate for the levels of resolution achieved in balances that are based on strain gauge technology, force-measuring devices based on electromagnetic force compensation which can have significantly higher levels of resolution require reference voltages that have a higher stability and less noise. Particularly complex and expensive measures are required for example to suppress the so-called popcorn noise which is typically caused by semiconductors that have metallic impurities in the transition zones.
An electronic balance with electromagnetic force compensation is described for example in [7], DE 33 24 402. The electrical part of this balance includes a reference voltage source with a Zener diode which is a determinant factor for the measurement of the compensation current. The properties of the Zener diode are temperature-dependent, and a corrective circuit is therefore used to compensate the temperature-related influence factors. Thus, in balances based on electromagnetic force compensation that are used with a high measurement resolution, not only the noise characteristics but also the temperature-related behavior characteristics of the reference or the reference unit are of special importance.